This invention relates generally to organic Rankine cycle plants, and more particularly to methods and apparatus for improving cold start performance of organic Rankine cycle plants.
Rankine cycles use a working fluid in a closed cycle to gather heat from a heating source or a hot reservoir by generating a hot gaseous stream that expands through a turbine to generate power. The expanded stream is condensed in a condenser by rejecting the heat to a cold reservoir. The working fluid in a Rankine cycle follows a closed loop and is re-used constantly. The efficiency of Rankine cycles such as Organic Rankine Cycles (ORC)s in a low-temperature heat recovery application is very sensitive to the temperatures of the hot and cold reservoirs between which they operate. In many cases, these temperatures change significantly during the lifetime of the plant. Geothermal plants, for example, may be designed for a particular temperature of geothermal heating fluid from the earth, but lose efficiency as the ground fluid cools over time. Air-cooled ORC plants that use an exhaust at a constant temperature from a larger plant as their heating fluid will still deviate from their design operating condition as the outside air temperature changes with the seasons or even between morning and evening.
Current ORC plant designs are problematic in that the working fluid(s) condense and settle down inside the loop after the plant shuts down and/or in front or after the expanders. Plant start-up is particularly difficult and may fail with the highly viscous fluid blocking the expansion machines during cold ambient temperatures.
In view of the foregoing, it would be advantageous to provide methods and apparatus for ensuring the start-up routine associated with Organic Rankine Cycle plants works properly for each start event.